1. Field of the Invention
The present invention relates to power-assisted steering systems in automotive vehicles, and more particularly to a power-assisted steering system of the type which includes a hydraulic reaction mechanism for applying a reaction force to the driver's steering effort in accordance with travel speed of the vehicle.
2. Description of the Prior Art
In Japanese Patent Publication No. 49-30659 there has been proposed a power-assisted steering system of this kind which includes a hydraulic reaction mechanism for applying a reaction force to the driver's steering effort in accordance with a steering load acting on the dirigible road wheels of the vehicle and travel speed of the vehicle. In such a conventional steering system, the reaction mechanism includes a reaction fluid chamber to be applied with a hydraulic pressure caused by operation of a changeover valve unit in turning maneuvers of the vehicle. The hydraulic pressure in the reaction fluid chamber is controlled to decrease in accordance with an increase of travel speed in the vehicle. For this reason, when the changeover valve unit is maintained in a neutral position, the hydraulic pressure in the reaction fluid chamber becomes approximately zero, resulting in instability of the steering wheel in its neutral position.